The present invention is generally related to a system for separating suspended solids from hydrocarbons and water found in the bottoms of petroleum tanks as sludge. The invention is more particularly directed to separating the solids as a dry non-hazardous filtrate whereby the hydrocarbons and water are extracted for treatment at a oil/water separator capable of stripping the hydrocarbons. The invention is most particularly directed to providing a system for hydrocarbon and solid separation wherein no additional water is used, therefore minimizing the volume of contaminated water to be treated to only that amount which is separated from the sludge in order to meet environmental guidelines and minimize treatment costs.
The invention provides a process for efficiently and safely filtering petroleum tank bottom sludge, especially those petroleum tanks for storage of so-called finished products, such as kerosene, diesel oil, gasoline or No. 2 fuel oil, rather than tanks holding heavy oil contaminated solids, particularly those formed as residue by-products from the treatment of oil contaminated water or crude petroleum tank sludge, wherein a first step of bulk separating in a gravity separator, settlement tank, or an API separator, is required. However, in treating finished products, a critical concern is the flashpoint of the particular petroleum product, which must be considered most importantly at the time that the filtered cake is dried for containerization. Hazards can arise in filtering finished product storage tank sludge due to the creation of a vaporous, explosive environment.
Historically, oil storage tanks were cleaned by simply depositing the sludge in a pit and covering it. This is obviously not an acceptable practice for protecting the environment. The most common ways of disposal are to haul the sludge for incineration off-site, transport it to a treatment plant, or dump it at a protected landfill site. Disposing of hydrocarbons without polluting is very costly. On the other hand, separating the hydrocarbons from the solids found in the sludge is advantageous to the extent that the hydrocarbons may be resold for other purposes. However, a process that is intended to perform the separation must be done efficiently and effectively in order to make the reclamation effort environmentally and economically worthwhile. If the hydrocarbons cannot be easily separated for sale as a by-product, there is more likelihood that the entire sludge material will be incinerated, or treated and disposed of, without thought of separating the hydrocarbons for resale.
The present invention provides for removing the hazardous waste from oil tank bottom sludge, so that it can therefore be classified as "special waste". This status would permit such material to be containerized and placed into a landfill under Environmental Protection Agency (EPA) guidelines. However, to be in this category, the water content must be low and no free liquid present. Therefore a dry cake must result from the process to bring the water content down to acceptable levels. The present invention is a significant improvement to existing methods that involve hauling away sludge for incineration or to alternate disposal sites. A 90% cost saving per unit gallon of sludge can be attained over these prior disposal techniques. Also, by achieving a dry cake filtrate, which meets EPA guidelines, disposal permits, such as are required for dumping in most landfills, are more quickly obtained due to the safe quality of the cake to be disposed.
Known filtering techniques also employ so-called filter aids for precoating a filter in order that clogging does not occur from the solids accumulating on filtering sieves or plates. Usually, these known techniques for filtering heavy oil sludge involve adding a diluent, or have a heating step required. These processes often require the infusion of additional water, solvents, or other materials, to the sludge to be filtered. Additionally, in prior art processes that utilize filter aids, clamped filter presses, and the like, have been employed without addressing the vaporization/explosion hazard found with low flashpoint products, such as gasoline sludge.
Solvent washes, water washes and other means for cleaning filters during hydrocarbon separating steps are known. Compressed air, and other gases, are forced through filters to dry the solids following the solvent wash in many methods also found in the art. In U.S. Pat. No. 4,260,489, a method for treating oily sludge is disclosed which comprises filtering the sludge through a plate, leaf-and-tube, or candle, filter, that is operated under pressure. A filter aid, which may be diatomaceous earth, fly ash, or powdered polymers, may be added to the sludge before filtration to ensure longer and improved filtration by increasing the porosity of the filter cake and reducing the differential pressure per unit cake thickness, while preventing the cake from blinding. A drying step utilizing cold or hot air after the filtration is also made. This process also includes the use of a solvent to remove the hydrocarbon from the heavy oil. A water mix for a precoating step is also provided. The more costly and environmentally undesirable use of a diluent is required for use with a precoated surface filter by treating the filtration residue with a light hydrocarbon solvent and/or steam stripping the extracted residue.
U.S. Pat. No. 3,835,021 provides for the disposal of refinery waste sludge by filtering the sludge through a filter press that is precoated with diatomaceous earth. The resulting filter cake is washed out with water. Similarly, U.S. Pat. No. 2,430,182 discloses a process for treating water/oil refinery waste sludge by means of filtering the sludge with a filter that is also precoated with diatomaceous earth.
U.S. Pat. No. 4,774,007 involves a process for cleaning an oil waste pit and removing the hydrocarbon-contaminated matter using a liquid return process to repeatedly supply a part of the recovered filtrate back to the contaminated waste until the waste is substantially contaminate-free. Processes for filtering a solid from a liquid are generally shown in U.S. Pat. Nos. 2,696,306 and 2,971,648 in which diatomaceous earth is added as a filter aid. Other conventional sludge filtration systems are shown in U.S. Pat. Nos. 4,774,022, 4,687,584 and 4,310,422. In U.S. Pat. No. 4,310,422, a filter cake, which builds up during the course of continuing filtration, and which is primarily composed of the filter aid, is dried by flushing with inert gas.
My present invention overcomes the problems and inefficient procedures known in the prior art and is primarily directed towards the hydrocarbon reclamation from petroleum storage tanks. The invention does not require heating of the sludge or the use of a diluent. No additional water need be added to the sludge, which is an environmentally beneficial feature, since the resulting hydrocarbon/water mix will include the minimal amount of water that has to be treated or disposed. Moreover, because no solvent wash is required, another pollution concern is eliminated. The present invention also eliminates a precoating step for the filtering device and does not require monitoring or adding filter aid during filtering. The filter aid is preferably diatomaceous earth which is introduced without requiring it to be pre-mixed with a third medium. For treating the petroleum/finished product tank sludge, a sealed multi-plate and paper immersion type filter is employed in my process whereby flashpoint hazards are substantially eliminated.